Numerous diagnostic devices have recently been introduced for use by relatively unskilled personnel. One such device is described in U.S. Ser. No. 106,757 filed Oct. 8, 1987. The system described is a flow-through device with having a porous support and an absorptive layer. A binder is immobilized on a test area of the porous support.
In use sample and assay reagents are allowed to flow through the porous support to the absorptive layer. The presence or absence of a visible signal on the test area indicates the presence or absence of analyte in the sample. The preferred device shown has a triangular test area surrounded by a background area of the porous support. The background area is desirable so that a visible signal on the test area contrasts from the unreacted background area.
Other devices have been introduced with reagents coated in patterns such as round dots, bars resembling a "minus" sign, and crosses resembling a "plus" sign. Some devices have multiple reagents immobilized on the support. For example some have positive controls, negative controls, or both.
These devices all have in common a support having one or more reagent immobilized at or near a surface. The surface bearing the reagent is then assembled in the device so that a fluid sample and assay reagents are deposited on the coated surface during the assay. Many of these devices also have one or more reagent coated in a distinct pattern. In each case the portion of the support bearing the immobilized reagent has a specific position within the test device. Where the device has multiple reagents immobilized, the reagents are positioned precisely in relation to each other and the device in distinct patterns.
Locating the reagents on the supports in the correct position and shape is difficult to accomplish in a cost effective manner. Manual spotting is time consuming and labor intensive. When the reagents are manually spotted, rejection rates are generally high because of unacceptable variations in spot location, concentration, and shape. The devices need to be equivalent to each other for quality assurance. This is particularly important for devices to detect viral antigens or antibodies to viral antigens such as HIV.
One system to facilitate correct placement of the reagent is described in U.S. Ser. No. 106,075 filed Oct. 8, 1987. In that system the binder is spotted in admixture with a marker so that the location of the binder can be detected when spotting a control reagent and when assembling the support into the device. The preferred device incorporates fluorescent dyes allowing quality control of devices prior to final assembly thereby reducing waste.
Another proposed solution to the problem of coating reagents is described in U.S. Pat. No. 4,748,042. That system uses "means for forming a transferable pattern" to transfer an antibody solution to a protein binding membrane. The particular means described is a foam pad. According to the patent the amount of fluid to be pumped onto the head is determined empirically by observing the quality of the markings on each membrane. When the quality has deteriorated to a predetermined level of poor quality new fluid is injected into the head. The patent recommends incorparating a dye so that an assembly operator can inspect the membranes. See col 3, lines 55-62 and col 4, lines 27-33. This design inherently suffers from concentration variations as the transfer surface is depleted of fluid. Another problem with the design is controlling the pressure with which the transfer surface contacts the membrane. The patent emphasizes the problem of damaging delicate membranes so that they are not useful in diagnostic devices. Nonetheless, the transfer surface must contact the membrane with sufficient force to transfer the antibody solution. As the transfer surface wears and as the volume of fluid on the transfer surface varies, the contact pressure of the transfer surface will be very difficult to control.
Accordingly, a need exists for an inexpensive, automatible system to locate precisely measured amounts of one or more reagents on a support and to locate precisely the immobilized reagent(s) within the assembled device.